The invention relates to an electronic circuit for safe forwarding of an analog electrical signal value (hereinafter called simply signal for short) between two or more systems communicating with one another. The signal may for instance be the representation of a physical variable (such as pressure, temperature, brightness, conductivity, etc.) or a technical variable (such as fill level, flow rate, set-point or actual values of a controlling value or closed control loop, etc.). The conversion of the physical or technical variable (hereinafter called variable for short) into a signal representing it is done by sensor or detector systems (hereinafter called transmitters for short) on-site, at the place (“in the field”) where the variable is to be detected or ascertained (the terms field devices or field transmitters are therefore often also used). The signal thus ascertained is then made available for further processing to other systems, sometimes even quite far away (displays, closed-loop control systems, open-loop controllers, motors, valves, and so forth, hereinafter called evaluation systems for short).
Manifold demands are made in terms of the performance and operating safety of such systems; some of these have been written down in the form of standards. These demands will be sketched out briefly below:
Current signal per NAMUR:
One standard that is widely used in industry for forwarding signals of physical or technical variables is an electrical current loop in accordance with NAMUR recommendations NE006 (or DIN IEC 60381, Part 1) and NE043 (NAMUR is an international association of users of automation technology in the processing industry). In it, an electrical loop current of between 4 mA and 20 mA flowing in a current loop represents the value of the physical or technical variable. Because of drifting and imprecision and the detection of regional overflows, a somewhat greater current range is permitted for representing the variables: 3.8 . . . 20.5 mA. Currents of less than 3.6 mA or greater than 21 mA should no longer be interpreted by the evaluation units as representing the physical or technical variable but rather as erroneous information from the sensor. A loop current of less than 3.6 mA or greater than 21 mA will therefore hereinafter briefly be called an fault current.
Field devices that have a current requirement of only less than 3.6 mA are often supplied with energy from the current loop itself. In that case, the term 2-conductor devices is used, since for connecting the devices, a line with only two wires is needed. The associated counterpart (evaluation system) must in this case make a suitable electrical power supply available for supplying the current loop. The field device in this case functions as a passive current sink.
Field devices with higher current consumption must be supplied with additional auxiliary energy (3-conductor or 4-conductor devices). Besides the operating mode as a current sink (as in the case of 2-conductor devices), however, these devices can also act as an active current source. The electrical power supply of the current loop is then accommodated in the field device itself. The advantage here is that the connection to the evaluation systems can be done more simply (an electrical power supply is no longer necessary).
HART Protocol:
A purely analog electrical current signal cannot ever be used for more than transmitting a variable in one direction (for instance from the transmitter to the evaluation device). In many cases, however, it is desirable to transmit still further additional information, and the information flow of additional information should be possible in both directions. Such additional information includes for instance information on inspection conditions of the sensor, internal states of the sensor (such as temperature, operating voltage, etc.), calibration and parametrizing information of the sensor, and so forth. One widely used method for transmitting such additional information is to superimpose an FSK (Frequency Shift Keying, Bell Standard 202) on digital electrical information via the actual analog electrical signal. To that end, a higher-frequency signal is therefore superimposed on the low-frequency (quasi-steady-state) current signal with an upper 3 dB limit frequency of 25 Hz and can be sampled back and forth between the frequencies of 1200 Hz (representing a “0”) and 2200 Hz (representing a “1”). The mean value of the low-frequency analog current signal remains unchanged. So that field devices and evaluation systems from different manufacturers can exchange this additional information with one another, a uniform protocol for information exchange has been defined: the HART protocol (HART=Highway Addressable Remote Transducer), which is administered as a public protocol by the HCF (HART Communication Foundation), and in which certain minimum demands in terms of the (hardware) quality of the signals are also firmly defined.
Explosion Protection by Intrinsic Safety:
Particularly in the processing industry, physical or technical variables must often be measured or ascertained in areas where there is a potential risk of explosion. By suitable provisions in the field devices and evaluation systems (such as voltage and current limitation), the electrical energy in the signal to be forwarded can be limited in such a way that this signal cannot trip an explosion under any circumstances (short circuit, interruptions, thermal effects, etc.). In this case, the term used is intrinsically safe signals under DIN EN 50020 or IEC 60079-11).
Functional Safety:
In many cases, the magnitude of the forwarded signal can have a substantial influence on safety-relevant decisions (such as temperature, pressure or fill level in a chemical process to be regulated that involves highly toxic, explosive, or otherwise dangerous initial or final products). In these cases, for the safe running of the process it is decisive that the forwarded signal actually correspond to the measurement variable. Devices and systems in safety-relevant applications must therefore meet special demands, which are written and defined among other places in the series of Functional Safety Standards (DIN EN 61508 and DIN IEC 61511). A central component of these standards is the classification of devices and systems with regard to their so-called Safety Integrity Level (SIL). Depending on the magnitude of the danger in a system, the devices and systems used there must have an SIL of from 1 (little danger) to 4 (extreme danger). Limit values for the failure rates and the reliability of the devices used in such systems are linked directly to the SIL.
Since evaluation systems (control systems, controllers, etc.) can know only the forwarded signals but not the associated variables to be measured, in safety-relevant applications they must depend on very high reliability of the signals, or else additional signals must be used to determine the correctness. Often in such cases, the variables to be measured are detected and transmitted in redundant fashion (two or more transmitters, each with their own signals). Then the evaluation systems, by comparison of the forwarded signals (for instance for agreement within predetermined limits), can determine the correctness of the signals. A disadvantage here is the considerable additional hardware expense (multiple transmitters, multiple signal lines, comparison devices, and so forth) as well as the increase in complexity of the evaluation (for instance, by how much may the signals be allowed to deviate from one another and still be considered to be the same?).
The only device in a current loop that can determine the agreement of the current flowing in the current loop with the physical or technical variable to be represented is the transmitter itself, since only to that transmitter are both values known. It is therefore advantageous if a transmitter can ascertain the agreement of the current signal with the variable to be represented with adequately high probability, and in the event of a deviation of the current signal can safely put it into one of the two fault states (<3.6 mA or >21 mA, per NAMUR).